Ceric ion initiation of methyl methacrylate from
poly(glycidyl azide)-diol
Hulya Arslan
a, Mehmet S. Eroglu
b, Baki Hazer
a,c,*aDepartment of Chemistry, Zonguldak Karaelmas University, 67100 Zonguldak, Turkey bDepartment of Chemistry, TUBITAK-Marmara Research Center, Gebze, 41470 Kocaeli, Turkey
cFood Science and Technology Research Institute, Gebze, 41470 Kocaeli, Turkey
Received 3 September 1999; received in revised form 22 February 2000; accepted 31 May 2000
Abstract
Polymerization of methyl methacrylate initiated by ceric ammonium nitrate in combination with poly(glycidyl azide)-diol was investigated in aqueous nitric acid. Poly(methyl methacrylate)-b-poly(glycidyl azide) copolymer was obtained. Block copolymer yield was increased using tetrabutyl ammonium hydrogen sulphate as a surfactant. The eect of nitric acid, ceric ions and diol concentration on the block copolymer yield was investigated. Block copolymers were characterized using GPC, 1H-NMR, FTIR, DSC, TGA and fractional precipitation methods. Ó 2001 Elsevier
Science Ltd. All rights reserved.
Keywords: Redox initiation; Ceric ammonium nitrate; Poly(methyl methacrylate)-b-poly(glycidyl azide); Tetrabutyl ammonium hydrogen sulphate
1. Introduction
Azide polymers have been synthesized during the last two decades to be used as an energetic binder and per-formance improving additive in the composite and composite modi®ed double-base propellant formula-tions. Poly(glycidyl azide)-diol (PGA-diol) is a typical example of this energetic azide polymers, which has the ability of self-decomposition even at a relatively low temperature and produces fuel rich decomposition products. PGA-diol is a low molecular weight and hy-droxyl terminated liquid prepolymer, which can be synthesized via the nucleophilic substitution reaction of its precursor, poly(epichlorohydrin)-diol (PECH-diol) with sodium azide (NaN3). PGA-diol contains
ener-getic pendant azidomethylene groups (±CH2±N3) on
the polyethere main chain and has a positive heat of
formation (957 kJ kgÿ1 at 293 K). The high
en-ergy potential and relatively low detonation sensitiv-ity properties allow the PGA to be considered as a monopropellant as well as polymeric binder and also to become a superior replacement for nitroglycerin in ei-ther crosslinked and uncrosslinked composite modi-®ed double-base propellants. Beyond superior thermal properties, this polymer has excellent physical±chemical properties such as low glass transition temperature (Tg ÿ50°C), low viscosity and high density compared
to the other widely used prepolymers in rocket propel-lant technology [1±6].
The block copolymers carrying PGA and various vinyl polymer segments are also expected to have promising applications as a binder in rocket fuel tech-nology. These types of polymers were previously syn-thesized by means of macroazonitriles having glycidyl azide moieties [7].
On the other hand, cerium(IV) ions are versatile re-agents for the oxidation of numerous functional groups in organic synthesis, as well as in transition metal chemistry. There are advantages of employing Ce(IV) as
*Corresponding author. Tel.: 372-323-3870; fax:
+90-372-257-4181/+90-372-323-8693.
E-mail address: hazer@karaelmas.edu.tr (B. Hazer).
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S0014-3057(00)00126-9
an oxidizing agent. Ceric ions are eective both by themselves as well as in association with organic sub-strates known as a reducing agents as redox systems for initiating vinyl polymerization. Alcohols, aldehydes, ketones, acids, amines, amides, thiourea, the compounds of acyl acetanilide and some natural polymers such as cellulose, glucose, maltose are known as reducing agents. In the case of alcohols, ®rstly ceric ions form a complex with alcohols and the formation of free radical occurs via decomposition of coordination complex. The most striking feature of redox process from a practical point of view is that it enables polymerization to be performed at a substantially faster rate than conven-tional methods. Also, it enables polymerization to be conducted at lower temperatures [8±11].
In this study, poly(methyl methacrylate)-b-poly(glycidyl azide) (PMMA-b-PGA) copolymers were synthesized by redox polymerization initiated by ceric ammonium nitrate in combination with PGA-diol in aqueous nitric acid.
2. Experimental 2.1. Materials
PGA-diol was synthesized by the nucleophilic sub-stitution reaction between pendant ±CH2±Cl groups of
PECH-diol and sodium azide (NaN3) following the
procedure described in the literature [5,12]. It has a number average molar mass (Mn) of 2200 g molÿ1 as
determined by vapour phase osmometry (VPO). The hydroxyl equivalent was determined as 0.94 meq gÿ1
according to the method cited in Ref. [13]. The viscosity and density of PGA-diol was 2:4 103 cP and 1.29
g cmÿ3 at 25°C, respectively. The chemical formula of
PGA is as follows:
Methylmethacrylate (MMA) was supplied from Fluka AG and freed from inhibitor by vacuum distilla-tion over calcium hydride.
Ceric ammonium nitrate (CAN), Ce(NH4)2(NO3)6
was supplied from Fluka AG and used as received. Tetrabutylammonium hydrogen sulphate (TBAHS) was used for increasing the solubilities of MMA and PGA-diol in the reaction system. These were purchased from Fluka AG with 97% purity and used as received.
Chloroform and methanol were used as solvent and nonsolvent in the fractional precipitation process, spectively. They were of reagent grade and used as re-ceived.
2.2. Synthesis of copolymers
The copolymerization initiated by ceric ion was car-ried out in a solution of PGA-diol, MMA and CAN in tetrabutylammonium hydrogen sulphate (TBAHS). For this purpose, a series of reaction mixtures containing PGA-diol, MMA and CAN having dierent PGA-diol/ MMA ratios were prepared in Pyrex tubes. In order to increase the solubility of CAN in the reaction mixture, approximately 10 mg of TBAHS was added to reaction tubes. After being capped with a rubber septa, each re-action tube was purged with nitrogen for 1 min and immersed to an oil bath at 40°C for a given time with continuous stirring. The reaction product was precipi-tated in methanol, collected by ®ltration and dried to constant weight. The data pertaining to the initial feed compositions and their corresponding copolymerization yield are given in Table 1.
In order to separate the block copolymers from crude copolymer mixture, the fractional precipitation method was used. The volume ratio of nonsolvent (methanol) to solvent (CHCl3), c, was calculated to separate pure
block copolymers from homopolymers [14]. For this purpose, approximately 0.5 g of polymer was dissolved in 10 ml CHCl3, and methanol was gradually added to
this solution until the polymer precipitated. Then, c value of the polymer was calculated by taking the vol-ume ratio of nonsolvent (methanol) to solvent (CHCl3).
2.3. Instrumentation
Gel permeation chromatography (GPC) was used to determine molecular weights and their distributions with a Waters instrument (410 Dierential Refractometer) in THF. The elution rate was 1 ml minÿ1. Waters Styragel
columns HR1 and HT6E were used, and molecular weights were calibrated with polystyrene standards (TOSOH Corporation). A Knauer type vapour phase osmometer was used to determine the Mnvalue of
PGA-diol prepolymer using benzyl (Mw 210:23 g molÿ1) as
calibration standard in tetrahydrofuran (THF) as sol-vent.
1H-NMR spectra of copolymers were recorded on a
Bruker-AC 200 L, 200 MHz NMR spectrometer using CDCl3 as solvent.
Dierential scanning calorimetry (DSC) and ther-mogravimetric analysis, TGA, measurements were car-ried out under nitrogen atmosphere by using a Dupont DSC-9100 and Dupont TGA-951, respectively, with a TA-9900 data processing system at heating rate of 10°C minÿ1.
The tensile mechanical properties of copolymers were measured on a Tensilon (UTM II) tester at room tem-perature with a crosshead speed of 10 mm minÿ1.
3. Results and discussion
PMMA-b-PGA copolymers were synthesized via re-dox initiation at 40°C in the presence of TBAHS. We propose the reaction mechanism shown below as a possible route of copolymerization [15,16].
After these reactions, PMMA-b-PGA block copolymers could be ABA type of block copolymers [17].
Pure block copolymers were isolated by fractional precipitation of homopolymers and copolymers using methanol as nonsolvent from chloroform solution of crude product. c-values of pure block copolymers were determined as volume ratio of methanol consumed for precipitation to chloroform solution. Fractional pre-cipitation result of products listed in Table 1 con®rms the formation of block copolymer structure. c for pure block copolymers were found between 2.2 and 2.7, while for homo-PMMA and PGA-diol were 0.8 and 3.3, re-spectively.
To observe the eect of diol concentration on the copolymer conversion, various feed compositions hav-ing dierent PGA-diol/CAN ratios were prepared and copolymerization was initiated. As shown in Table 1, an increase in the diol concentration of initial feed caused an increase in copolymer yield. This can clearly be fol-lowed by comparing the experiments especially in this order: 20,21,22 and 16,14,17. Acid concentration of initial feed also in¯uenced block copolymer yield. When we compare run nos. 16 and 20, in Table 1, the lower acid concentration causes higher PMMA-b-PGA block copolymer yield. Similarly, the same eect can be seen when comparing run nos. 14 and 21. The eects of nitric acid, ceric ions and diol concentration on block co-polymer yields were investigated and the same results obtained with that of the previous study [18].
For the spectroscopic characterization of copolymers IR and1H-NMR spectra of copolymers were recorded.
NMR and FTIR analyses of block copolymers also
Table 1 Results and condition s of the polymerizatio n of M MA by Ce IV/-d iol red ox syste m at 40 °C Run no. Polym erization initial feed compo sition Total polymer yield (g ) Blo ck copolym er fr actiona tion (wt.% ) 2: 2 < c < 2: 7 PGA-d iol (g) M M A H N O3 (mo ll ÿ 1)C e 4 (mo ll ÿ 1) (ml) (mol l ÿ 1) (ml) 14 2.793 4.65 3 3 0.05 3 3.943 47.3 15 3.409 BuM A 3 3 0.05 3 3.831 44.5 16 1.511 4.65 3 3 0.05 3 2.465 34.0 17 4.733 4.65 3 3 0.05 3 5.950 43.1 18 3.065 5.3 4 0.5 0.05 3 4.508 63.0 19 4.020 3.7 4 0.5 0.05 6 7.067 65.4 20 1.531 4.65 3 0.5 0.05 3 3.555 48.3 21 2.830 4.65 3 0.5 0.05 3 5.136 62.0 22 4.012 4.65 3 0.5 0.05 3 6.809 65.0
con®rmed the presence of both polymer segments. In the IR spectra of copolymers, the strong absorption peaks of ±CH2±N3 and ±C±O±C± groups pertaining to PGA
units were observed at 2100 and 1110 cmÿ1, respectively.
In this spectrum, a strong absorption peak observed at 1735 cmÿ1was due to C@O groups of MMA segments
of copolymers.
Fig. 1 exhibits the 1H-NMR spectrum of the
co-polymer sample of run no. 19 in Table 1, which shows the characteristic peaks of PGA and PMMA block segments of copolymer. Characteristic peaks for PMMA and PGA blocks were observed (d, ppm) at d 3:6 (due to ±OCH3 protons of MMA), d 1:9±2:0 (due to
±CH2± protons of MMA on the backbone), d 1:0±0:8
(due to ±CH3protons of MMA), d 3:4 (due to ±CH2±
N3 protons of PGA), d 3:6 (small shoulder which is
due to ±CH2±CH±O± protons on the main chain of
PGA). PGA inclusion of copolymers calculated from the NMR was collected in Table 2.
PGA content of copolymers was also determined by recording their TGA thermograms. In weight loss and derivative weight loss curves of the copolymer sample (run no. 19 in Table 1), two-step weight loss process is clearly observed (Fig. 2). As was reported earlier that the ®rst decomposition step at around 240°C corresponding
to nearly 13% weight loss is due to exothermic decom-position of side ±N3 groups of PGA blocks [2]. The
second weight loss process obtained at around 376°C is due to subsequent decomposition of PGA main chain and PMMA blocks. By taking into account that pure PGA has 36% weight loss at the characteristic ®rst stage [2], we can easily calculate the PGA content of copoly-mers. The composition of copolymers calculated from their ®rst stage weight loss is given in Table 2.
In DSC thermograms of copolymers (Fig. 3), the exothermic peaks at around 250°C coincide with the ®rst step weight loss of copolymers and peak areas are direct quantitative measures of their PGA content. It was previously reported that the heat liberated in the ®rst step decomposition of pure PGA-diol was 1828 J gÿ1[2].
The exothermic decomposition of side ±N3 groups of
PGA blocks corresponding to the ®rst stage weight loss process shown in Fig. 2 has been exploited to calculate the PGA content of the block copolymers (run nos. 18± 20 in Table 1). Hence, the ratio of energy liberated at the exothermic ®rst stage decomposition of copolymers to the energy value of pure PGA-diol makes it possible to calculate the copolymer compositions. The copolymer compositions calculated from the DSC measurements were collected in Table 2. As was observed from Fig. 3, the exothermicity of ®rst stage decomposition of
co-Fig. 1.1H-NMR spectrum of PGA-b-PMMA copolymer (run
no. 19 in Table 1).
Table 2
Molecular weights and polymer contents of the block copolymers
Run no. GPC analysis Block copolymer analysis PGA (mol%)
Mw 104 Mn 104 Polydispersity, Mw=Mn NMR TGA DSC 14 29.47 16.85 1.75 ± ± ± 15 43.52 23.20 1.87 ± ± ± 16 18.96 11.18 1.69 ± ± ± 17 28.88 14.79 1.95 ± ± ± 18 80.57 20.64 3.90 30 37 33 19 54.72 21.29 2.57 32 45 43 20 39.84 13.11 3.04 26 31 27
Fig. 2. TGA curves of PGA-b-PMMA copolymer (run no. 19 in Table 1).
polymers increased with increasing PGA contents, and the copolymer compositions calculated from TGA and DSC thermograms are in good accordance.
In order to understand the thermal behaviour of block copolymers well, their glass transition tem-peratures (Tg) were determined. For a typical block
copolymer dierent glass transition temperatures repre-senting those of corresponding homopolymers can be expected. However, in this work, due to the compati-bility of the block segments the block copolymers showed single glass transition temperature at a temper-ature interval of ÿ8, 0°C which is between their Tgs of
corresponding homopolymers. The glass transition temperature of PGA-diol is ÿ47°C, whereas that of PMMA is 105°C. This result showed that PGA blocks are miscible and compatible with PMMA blocks in co-polymer. Interestingly, the calculated Tg values from
Fox equations were between 34°C and 54°C.
Molecular weights and polymer contents of pure block copolymers were listed in Table 2. Molecular weights of pure block copolymers ranged from 18:96 104 to 80:57 104and their GPC chromatograms were
unimodal.
The tensile mechanical test results of block copoly-mers are given in Table 3. As is seen from this table, copolymers have a lower tensile stress and higher elon-gation values than homo-PMMA. The increase in PGA content of copolymers causes a considerable decrease in tensile stress and an increase in elongation values. This result is reasonable since pure PGA behaves as an elastomer at room temperature.
4. Conclusions
PMMA-b-PGA block copolymers having propellant binders properties can be successfully prepared by the redox initiators in high yields. This polymerization method enables us to insert PGA blocks into the block copolymers in desired amounts up to 45 mol%. PMMA-b-PGA block copolymers are also thermoplastic elas-tomers, and they can be used to prepare composite materials.
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Table 3
Tensile mechanical properties of PGA-b-PMMA block co-polymer ®lms
Run no.
Elonga-tion (%) Stress atbreak (MPa) Yield stress(MPa)
Homo-PMMA 5.4 68.5 ±
18 12.0 35.6 ±
19 81.0 17.3 14.6
